U.S. Pat. Nos. 5,071,157 and 5,096,219 describe real time controlled vehicle suspension systems that use eight primary sensors to sense wheel and body states for suspension control. The eight primary sensors include four position sensors, which measure relative vertical position between the vehicle body at each of the four vehicle wheels, and ,four accelerometers, which measure the absolute vertical body acceleration at each corner of the vehicle adjacent a vehicle wheel. The signals from these sensors are provided to a control which varies, in real time, the damping of a variable damper or actuator between each vehicle wheel and its corresponding body corner.
In each of these prior art systems, the desired force to be exerted by each damper or actuator is computed as a combination of the absolute vertical wheel velocity of the associated wheel and three independent absolute body modal velocities defining body motion with reference to an inertial frame of reference, such as absolute body heave, roll and pitch velocities. In the system of U.S. Pat. No. 5,071,157, for example, the body accelerometer outputs are integrated to obtain absolute body vertical velocity signals, which are geometrically transformed into absolute body heave, roll and pitch velocity signals. Each of the relative position sensor outputs is differentiated to obtain a relative body/wheel velocity signal for the respective wheel, which can be subtracted from the absolute body vertical velocity signal for the wheel to obtain an absolute vertical wheel velocity signal. The system of U.S. Pat. No. 5,096,219 differs from the preceding by first transforming the absolute body acceleration signals to absolute body heave, roll and pitch acceleration signals before integrating the latter to obtain the absolute body modal velocity signals but is otherwise similar in operation. In each case the required absolute body modal velocity signals are derived from body corner mounted accelerometers, which add significantly to the cost of the system.